Shooting fish in a barrel is the order of the day in comedian Bill Maher's faintly tiresome attack on religion - by which he largely means Christianity. Like a thousand media liberals before him, Maher journeys gigglingly through America's centres of redneck piety, and has a big laugh at their expense on the grounds that their belief system is crass, reactionary, inconsistent, tyrannical and fictional. Well, duh.

What Maher should have done was interview atheists themselves, the likes of Christopher Hitchens and Richard Dawkins, and investigate the ethical burden that atheism has to shoulder, the need to substitute a rational morality in place of superstition. Is there such a morality? One that we can all agree on?

Maher is often funny, particularly when he shows clips of his old standup material. (As the child of a Jew and a Catholic, he grew up confused and took a lawyer into confession: "Bless me father for I have sinned ... I think you know Mr Cohen.")

The acid test is whether you are prepared to hammer Islam. Maher passes, sort of. Though he never attacks any religion as fiercely as Christianity, he challenges a Muslim rapper to admit that Rushdie's death sentence was wrong. Maher's film only really catches fire, however, with his final monologue, condemning religion for being an "enabler" of violence, like a mafia wife. This should have been his starting point.